Theia Collision Might Have Seeded Precious Metals on Earth

For many years, scientists believed that the materials for life on Earth were seeded in a collision with the planetary body Theia, until that theory fell into disrepute in favor of an origin-of-life theory that depended on the Late Heavy Bombardment. Now, the debate is being revived in full force, as new evidence from a Stanford University indicates that Theia might be the source of our precious metals. If this is true, it may provide more insight into the origins of life on Earth.

The precious metals (gold and the six platinum group elements) exist in Earth's core and in the mantle in approximately the same proportions as in meteorites, leading scientists to believe that they came from an extraterrestrial impact. Some scientists hypothesized that the surface metals came from the collision with the planetary body Theia, which also formed our Moon, about 20 to 100 million years after our Solar System's formation. But that theory has somewhat fallen out of favor, with many researchers claiming that the metals came from asteroids in the Late Heavy Bombardment, a series of asteroid collisions with Earth that occurred significantly later than the Theia impact—approximately 4 billion years ago.

In this new study, published by Geochemistry, Geophysics, Geosystems, a journal of the American Geophysical Union, this "veneer" of precious metals could plausibly have come from Theia. Lead author Norman Sleep, a geophysicist at Stanford, analyzed Theia's size and estimates of its iron contents in order to evaluate whether the object could be responsible for the iron concentrations on the Earth and Moon, and then determined how likely it was that large enough impacts would occur during the Late Heavy Bombardment that the asteroids could have seeded our precious metals.

According to Sleep's calculations, the amount of iron in Theia's core could account for the concentrations of precious metals in Earth's mantle. The excess oxygen in Earth's surface would have bonded with the iron, which would have meant that the iron couldn't dissolve the platinum metals so they would sink into Earth's core. However, his calculations for the Late Heavy Bombardment also checked out, so this study doesn't resolve the debate so much as re-ignite it.

"The paper is not intended to be last word on the subject, but rather show that both alternatives still appear to be viable, and that more geochemical work needs to be done," Sleep said in an AGU statement.

If the Theia collision theory is true, then it could tell us how early life arose on Earth. If the impacts in the Late Heavy Bombardment were severe enough to leave precious metals in the surface, then they might have been capable of destroying any life that had begun prior to that event. But if the Theia collision theory turns out to be true, then the LHB impacts were likely less severe and life would have formed much earlier in Earth's history.

"If the Earth got its late veneer from several large impactors during the Late Heavy Bombardment, if life had already evolved at that point, these impacts could have killed it off," said Rebecca Fischer, a geophysicist at the Smithsonian National Museum of Natural History. "Or, alternatively, the impacts could have killed most existing life but left only those organisms adapted to extreme heat environments, such as bacteria that live in hydrothermal vents."